What is the greatest common factor?
How do you know when you have found the greatest one?
To find the greatest common factor, first list the factors of both numbers, then find which one is the greatest shared factor.
For example, to find the greatest common factor of 20 and 30:
Factors of 20: 1, 2, 4, 5, 10, 20
Factors of 30: 1, 2, 3, 5, 6, 10, 15
The common factors of 20 and 30 are 1, 2, 5, and 10
The greatest common factor of 20 and 30 is 10